A far infrared ray is light having a wavelength of 8 um to 12 um, is emitted from a human or an animal as heat, i.e., an infrared ray, and is therefore used to capture an image at dark place and observe a temperature distribution. For example, it is used for a field such as night viewer (night vision) in an automobile, nightwatch and a power supply control of an air conditioner, a television, etc.
In an application where a shape of an object to be captured (subject) is image-formed such as thermoviewer and a night vision system, a relatively high resolution is needed.
In the future, in order to widen the applications of a far infrared device, an optical system should be produced at low costs while such a high resolution is kept.
Also, in order to improve sensitivity, an infrared transmittance of the optical system should be improved.
FIG. 10 illustrates a configuration of an infrared optical system included in an infrared image capturing apparatus in the related art.
It is supposed that an object to be captured is disposed at a left side in FIG. 10.
As shown, the infrared optical system in the related art includes an aperture stop 104, a first lens 101, a second lens 102 and a sensor window 103 from an object side. “Simg” disposed at a rightmost side in FIG. 10 represents an image capturing surface of an image sensor (imager) for detecting an infrared image.
The first lens 101 functions as a light collecting lens for collecting infrared rays from an object.
Specifically, the first lens 101 in this case is a so-called plano-convex lens having an aspherical convex surface at the object side and a plane surface at the image surface side, and collects infrared rays emitted from each position of the subject on the imager.
The second lens 102 functions as an aberration correction lens for correcting a variety of aberrations such as spherical aberration, coma aberration and astigmatism aberration. In this case, the second lens 102 has an aspherical shape at the object side and the plane surface at the image surface side as shown.
The sensor window 103 is disposed to protect the image capturing surface Simg of the imager.
Here, for the optical system for collecting far infrared rays, a glass lens used for general visible light cannot be used because the transmittance is low. A suitable material well transmits infrared rays and includes germanium, silicon, zinc sulfide, selenium sulfide, chalcogenide glass provided by mixing chalcogen and germanium.
Germanium has a high refractive index, i.e., about 4, helps to aberration correction and less absorbs infrared light, which has been conventionally widely used. However, as germanium is a rare mineral, the material itself is expensive. When the number of the lenses increases, the image capturing optical system becomes undesirably very expensive.
Although the aspherical shape can be used to provide the aberration correction with the minimum number of the lenses, cutting work and polishing processing are also costly. At present, an inexpensive infrared optical system cannot be configured.
The above-described materials other than germanium are inexpensive materials as compared to germanium. However, working and processing remain costly. An inexpensive optical system is not yet achieved.
It is desirable that the resin lens can be produced inexpensively by molding at low costs.
However, most of the resin lens does not transmit electromagnetic rays at far infrared range. Polyethylene resin is used to a small extent.
Note that it is not true that polyethylene does not absorb. When a general lens shape is tried to be provided, it transmits almost no infrared rays.
At present, the resin lens is not used for image-forming that requires high resolution such as the above-described thermoviewer and a night vision system, but is used for simply determining the presence or absence of a heat source such as a motion sensor.
For example, Patent Document 1 discloses a configuration that a dome shaped region segmentation lens (a region segmentation prism) is used to segment a special region to determine the presence or absence of a human or a material emitting infrared rays in each region.
In addition, Patent Document 2 and Patent Document 3 disclose an optical system using a Fresnel lens.